Technical Abstract:
A total of 480 switchgrass genotypes, belonging to 71 accessions, were evaluated for variation in 19 SSR markers and cpDNA sequences at five introns or intergenic regions. Approximately 75% of the individuals were unequivocally classified as upland or lowland individuals, based on phenotype, genotype (SSR markers), and cytotype (cpDNA sequences). The remaining 25% of individuals represented a wide range of unusual individuals with various combinations of phenotype, genotype, and cytotype. Each of these individuals represents some level of ancient gene flow; 46 of these individuals represent the strongest evidence for hybrid origin and ancient gene flow (Figure 1). The various combinations of phenotype, genotype, and cytotype reveal bidirectional gene flow between upland and lowland ecotypes, between tetraploid and octoploid chromosome levels, and different levels of gene flow indicative of ancient hybridizations of differing ages, occurring over an extremely long period of time. The primary center of diversity for switchgrass is represented by remnant patches of prairie and savanna in the eastern Gulf Coast region, extending northward along the Atlantic Seaboard (Figure 2). Many of the southeastern sites are likely ancient remnants that served as prairie and savanna refugia during major ice age events. Much of the variation present in the eastern Gulf Coast region was preserved along the Atlantic Seaboard, largely due to the relatively mild climate change along this latitudinal gradient. Molecular clock computations suggest that the earliest upland-lowland divergence and the earliest transitions from tetraploids to octoploids occurred approximately 1.5 to 1 M ybp. Repeated ice age cycles, leading to cyclic migrations between northern prairie and savanna sites and southern refugia, have preserved massive amounts of genetic variability within individual polyploid genotypes, within local habitats, and across a broad geographic landscape. Much of this variability is available for use in improving switchgrass as a dedicated bioenergy crop.